1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to a diffractive optical element having such a grating structure that rays of a plurality of wavelengths or rays of a specific wavelength band concentrate at a specific order (a design order) of diffraction, and to an optical system having the diffractive optical element.
2. Description of Related Art
Heretofore, as one of methods of correcting chromatic aberration of an optical system, there is known a method of combining two glass (lens) materials which differ in dispersion.
In contrast to the method of reducing chromatic aberration by combining glass materials, there is known another method, which is disclosed in the optical literature, such as "International Lens Design Conference (1990)", SPIE Vol. 1354, etc., and the specifications of Japanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI 6-324262 and U.S. Pat. No. 5,044,706. In the case of that method, chromatic aberration is corrected by means of a diffractive optical element which is provided with a diffraction grating for a diffracting action and is disposed on a lens surface or a part of an optical system. That method is based on a physical phenomenon that the direction in which chromatic aberration arises for a ray of light of a reference wavelength becomes opposite between a refractive surface and a diffractive surface in an optical system.
Further, the diffractive optical element of such a type can be arranged to produce an advantageous correcting effect, like an aspheric lens, on the aberration by varying the period of a periodic structure of its diffraction grating.
Here, compared with a refracting action of rays of light, while one ray of light remains one even after refraction at a lens surface, one ray of light is split into rays of a plurality of orders after diffraction at a diffractive surface.
Therefore, in using a diffractive optical element for a lens system, it is necessary to decide the grating structure in such a way as to cause a light flux of a useful wavelength region to concentrate at a specific order (design order) of diffraction. With the light flux concentrating at the specific order, rays of diffraction light other than the light flux of the specific order have a low degree of intensity. When the intensity becomes zero, the rays of diffraction light would not exist.
In order to attain the above-stated feature, the diffraction efficiency of a ray of light of the design order must be sufficiently high. Further, in a case where there are some rays of light having diffraction orders other than the design order, these rays are imaged in a place different from the imaging place of the ray of light of the design order, and thus appear as flare light.
For an optical system using a diffractive optical element, therefore, it is important to pay sufficient heed to the spectral distribution of diffraction efficiency at the design order and the behavior of rays of diffraction light of orders other than the design order.
FIG. 11 shows a case where a diffractive optical element 1, which has a diffraction grating 3 and is composed of one layer on a base plate 2, is formed on a surface of an optical system. In this case, diffraction efficiency for a specific order of diffraction is obtained as shown in FIG. 12, which shows the characteristic of the diffraction efficiency. In FIG. 12, the abscissa axis of a graph indicates wavelength (nm) and the ordinate axis indicates the diffraction efficiency (%)
The diffractive optical element 1 is designed to have the diffraction efficiency become highest at the first order of diffraction (shown in a full line curve in FIG. 12) in the useful wavelength region. In other words, the design order of the diffractive optical element 1 is the first order.
Further, FIG. 12 shows also the diffraction efficiency of a diffraction order near the design order, i.e., zero-order light and second-order light (1.+-.1 order).
As shown in FIG. 12, at the design order, the diffraction efficiency becomes highest at a certain wavelength (540 nm) (hereinafter referred to as a "design wavelength") and gradually decreases at other wavelengths. The lower portion of the diffraction efficiency obtained at the design order becomes diffraction light of other orders and comes to appear as flare light. Further, in a case where the diffractive optical element is provided with a plurality of diffraction gratings, a drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
Diffractive optical elements having the structure capable of lessening the drop in diffraction efficiency are disclosed in Japanese Laid-Open Patent Applications No. HEI 9-127321, No. HEI 9-127322, etc. According to the structural arrangement disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321, the diffractive optical element is formed by laminating two layers 4 and 17 as shown in FIG. 13 which is a sectional view.
On the other hand, according to the structural arrangement disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322, the diffractive optical element has such a grating structure that three layers 4, 17 and 6 are laminated as shown in FIG. 14. The thickness of the layer 17 which is interposed in between diffraction grating surfaces 7 and 8 each of which is formed at a boundary face between two layers is not uniform. Thus, the diffractive optical element has the diffraction grating surfaces 7 and 8 which are formed at boundary faces between different materials. A high diffraction efficiency is attained by optimizing a difference in refractive index between the materials of layers disposed across each boundary and the depth of grating grooves formed in these layers.
In the above-stated diffractive optical element having a grating structure composed of a plurality of laminated layers, it is necessary to make at a desired value a wavelength characteristic of a difference in refractive index between the layer materials disposed across (in front and in rear of) a diffraction grating surface. For example, in the arrangement disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321, one of the layers disposed across the diffraction grating surface must be made of a material which is of a high refractive index and a low dispersion while the other layer must be made of a material which is of a low refractive index and a high dispersion. Use of materials for these layers is thus limited to this combination of different materials.
Further, in the case of the arrangement disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322, the kinds of layer materials are increased to three kinds. The number of kinds of selectable materials can be increased by varying the depth of grating grooves of the diffraction grating surfaces 7 and 8. The selectable layer materials are, however, inevitably limited as long as the wavelength characteristic of a difference in refractive index between these layer materials is used in arranging the diffractive optical element.
On the other hand, the materials usable across a boundary between layers are limited, from a manufacturing viewpoint, to such materials that have good adherence to each other, nearly the same coefficients of thermal expansion, and so on. It is also necessary that these materials must excel in workability for forming diffraction gratings.
The usable materials are thus limited not only in respect of optical characteristics but also from the manufacturing viewpoint. Therefore, it is not easy to find such optical materials that satisfy all of these conditions.